Page 340 - Vibrational Spectroscopic Imaging for Biomedical Applications

314 Cha pte r T e n

alumina femoral heads retrieved after medium-term implantation in
human body (i.e., 6 years and 8 months and 8 years and 2 months, in
Fig. 10.6c and d, respectively). Such a complex pattern of residual
stress is clearly affected by microdisplacements occurring between
the ceramic bearing surfaces. On the other hand, two areas of strong
compressive stress were found in the femoral head exposed in vivo
for 19 years (Fig. 10.6e). The general trend in surface residual stress in
the wear-zone surface of retrieved balls was increasingly tensile up to
several years exposure in vivo, then for implants subjected to longer
exposure times in human body residual stress fields in the wear zone
first of mixed tensile/compressive nature, and then progressed
toward fully compressive trends. The topographic location of areas of
stress intensification was not the same for all the retrievals and this
was considered to be the consequence of different designs of the arti-
ficial joints, different attitudes of the patients, and different angular
inclinations selected by the surgeon in positioning the ceramic ace-
tabular cup. Nevertheless, surface residual stress fields showed a
trend whose origin should reside in the mechanical interaction
between the bearing surfaces. Based on the experimental visualiza-
tion of stress patterns by Raman PS, we propose that both shock and
impingement of the acetabular cup on the femoral head introduce on
the ceramic surface a residual stress field whose nature changes from
tensile in the short term to compressive in the long-term exposure in
vivo and whose highest magnitude is reached after significant long-
term exposures. The pattern of residual stress can be referred to as the
loading history of the implant. A simplified model for explaining the
time dependence of surface residual stress in femoral heads can be then
given as follows. In the early period of implantation time, the surface
of the femoral head is subjected to significant local shocks and
impingement arising from severe microseparation phenomena tak-
ing place in the hip joint. As a consequence of such a micromechani-
cal situation, intergranular microcracking will take place and will
later develop into debris formation in main-wear zones. Cracking,
which is a consequence of local shocks and point forces, can intro-
duce in the surface a residual stress field of tensile nature. Cracks
selectively develop at the alumina grain boundaries where the stress
intensification is higher. Microcracking will successively develop into
grain detachment with subsequent development of a significant
amount of ceramic debris. This stage is likely accompanied by a
release of the tensile residual stress field, while the compressive
residual stresses stored on the ceramic joint surface are the conse-
quence of long-term impingement assisted by the presence of third
bodies (i.e., the ceramic debris). This latter compressive stress field
continuously increases with increasing exposure time in vivo up to
a saturation value, above which more extensive grain detachment
occurs and abraded areas (i.e., stripe-wear zones 46,47 ) may develop.